Communication pubs.acs.org/JACS
Synthesis of Highly Twisted and Fully π‑Conjugated Porphyrinic Oligomers Satoru Ito,† Satoru Hiroto,*,† Sangsu Lee,‡ Minjung Son,‡ Ichiro Hisaki,§ Takuya Yoshida,⊥ Dongho Kim,*,‡ Nagao Kobayashi,*,⊥ and Hiroshi Shinokubo*,† †
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Department of Chemistry, Yonsei University, Seoul 120-749, Korea § Department of Material and Life Science, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan ⊥ Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan ‡
S Supporting Information *
We attempted oxidative fusion of aminoporphyrin 3a with four mesityl groups, which was prepared through nitration and reduction of 1a. Oxidation of 3a with DDQ (2.0 equiv) in CHCl3 at room temperature afforded fused dimer 4a in 4% yield. After extensive optimization, we found that the oxidation of 3a in α,α,α-trifluorotoluene/trifluoroacetic acid (TFA, 20 equiv) afforded 4a in 97% yield (Scheme 1). None of the desired product was obtained without TFA. This protocol was also applied to 3b with sterically more demanding substituents to furnish 4b in 67% yield. Notably, this procedure enables the synthesis of highly congested diporphyrins 4a within 10 min even at room temperature.
ABSTRACT: Highly twisted π-conjugated molecules have been attractive but challenging targets. We report here an efficient synthesis of highly twisted diporphyrins with 126° and 136° twist angles that involves an oxidative fusion reaction of planar aminoporphyrin precursors at room temperature. Repeated amination−oxidative fusion sequences provide a unidirectionally twisted tetramer. The twisting angle of the tetramer is 298°. hree-dimensional (3D) π-conjugated molecules have received extensive attention in many chemical research areas due to their interesting structure and unique characteristics.1−3 Since the discovery of buckminsterfullerenes, synthetic chemists have devoted considerable effort to develop bottomup approaches to 3D π-conjugated molecules. Construction of 3D structures generally requires harsh conditions, such as laser irradiation, flash vacuum pyrolysis, and photochemical processes, to overcome their high strain energy.4 However, these protocols could not apply to π-conjugated functional molecules such as porphyrins, which undergo decomposition on heating and light irradiation. Aniline oxidation is a classic methodology for linking aromatic amines. However, the synthetic usefulness of this method has been overshadowed because of uncontrollable reactions to form a mixture of fused oligomers.5 In contrast, we have found that the oxidation of 2-amino-5,10,15-triarylporphyrins proceeded selectively to provide planar pyrazine-fused diporphyrins in high yields in one step.6 Herein, we report the synthesis of highly twisted diporphyrins and tetraporphyrin with very large twisting angles (136° and 298°) from planar 2aminoporphyrin precursors with more bulky substituents. Highly twisted molecules, particularly π-systems with 3D character, remain attractive but challenging targets.7 Twisting πsystems lead to the emergence of intriguing characteristics such as chiroptical properties and anisotropic crystal packing. The twisting approach to 3D π-systems has been investigated in the synthesis of oligoacenes, referred to as “twistacenes”. The maximum edge-to-edge twisting angle of the longest twistacene is 144°.7j However, additional systems with greater twist angles are needed if the specific features that are induced by molecular twisting are to be explored in detail.
T
© XXXX American Chemical Society
Scheme 1. Oxidative Dimerization of Nickel(II) 3Aminoporphyrins 3 to Diporphyrins 4a
a (a) LiNO3 (2 equiv), CHCl3/AcOH/Ac2O, 75 °C, 15 h for 1a; LiNO3 (4 equiv), CHCl3/AcOH/Ac2O, 55 °C, 4 h for 1b. (b) Sn (excess), conc. HCl, CHCl3, 60 °C, 1 h for 2a; Sn (excess), conc. HCl, CHCl3, 60 °C, 3 h for 2b. (c) DDQ (2 equiv), TFA (20 equiv), PhCF3, RT, 10 min for 3a; DDQ (2 equiv), TFA (20 equiv), PhCF3, RT, 2 h for 3b.
Received: November 20, 2014
A
DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society X-ray diffraction analysis of the products revealed that the present oxidative fusion reaction transformed 2D planar precursors into 3D twisted π-systems. The structures of the fused diporphyrins 4a and 4b are shown in Figure 1 and Figure
Figure 1. X-ray crystal structure of dimer 4a (a) side and (b) front views. Mesityl groups are omitted for clarity. The thermal ellipsoids are scaled at 50% probability level.
S11, respectively. Interestingly, both 4a and 4b exhibit highly twisted conformations in which the central pyrazine ring is also twisted. The twist angles of the pyrazine rings in 4a (13.01(9)°) and 4b (13.18(8)°) are nearly the same. While 1a is completely planar (Figure S10), the porphyrin units in 4a and 4b adopt ruffled conformations. The two porphyrin units are twisted in the same direction, and the edge-to-edge twisting angles in 4a and 4b are 125.6(8)° and 136.3(7)°, respectively. Meanwhile, 4a and 4b demonstrate unusually high solubility even in hexane.8 This fact demonstrates that deformation of πsurfaces leads to significant enhancement of solubility. This is remarkable since porphyrin oligomers typically suffer from low solubility. Different dynamic behaviors between 4a and 4b were observed in 1H NMR analysis. At room temperature, 4a exhibited two broad signals due to protons at the 3- and 5positions of the mesityl groups. On the other hand, four aryl proton peaks were observed in 4b. This difference is likely due to the rapid twist inversion of 4a compared with the relatively stable twisting of 4b at room temperature. In principle, the aryl proton signals should appear as four peaks in the twisted conformation. However, these peaks are interconvertible and merged into two peaks by twist inversion. This hypothesis was confirmed by variable temperature NMR analysis; four aryl proton signals in 4b became broader at higher temperatures and coalesced at 100 °C (Figure S5). Because of the high rotation barrier of mesityl groups, the signal broadening can be attributed to the helicity inversion.9 In 1H NMR spectra, pyrrole proton signals of 4b were upfield shifted as compared to 4a, while chemical shifts of monomers 1a and 1b were almost the same. This indicates that the π-twisting significantly influences the ring current of the porphyrin unit. The lower NICS(0) values of 4a than 4b also support substantially weakened aromaticity of the porphyrin ring (Figure S15). We expected that the subtle effect of molecular twisting could be detectable on the basis of change in interporphyrinic interactions in the twisted diporphyrins.10 Figure 2 shows the UV−vis absorption spectra of 1a, 1b, 4a, and 4b in CH2Cl2. Both dimers 4a and 4b exhibit bathochromic shift and broadening of the spectra compared with the corresponding monomers 1a and 1b, which indicates effective expansion of πconjugation despite their largely twisted structures.11 Compared with 4a, 4b shows slight bathochromic shift (51 cm−1) of the lowest energy band.12 The spectrum of 4a was further assigned by magnetic circular dichroism (MCD) spectral analysis. MCD signals for two bands at 622 and 585 nm
Figure 2. Magnetic circular dichroism (MCD) (top) and UV−vis absorption (bottom) spectra of 1a (black dash line), 1b (red dash line), 4a (black solid line), 4b (red solid line), and 7 (blue solid line) in CH2Cl2. The inset displays the enlarged absorption spectra.
showed oppositely signed Faraday B terms, which indicate the presence of two differently polarized transitions. Considering their optically forbidden character, these bands can be assigned as split Q-bands directed to two different axes (long and short axes, respectively). This type of separation of two Q-bands is often observed for fused mulitiporphyrins with effective πconjugation.13 To gain further insight into their electronic transitions, electrochemical analysis was performed. The results of cyclic voltammetry for 4a and 4b are summarized in Table S1. Both compounds showed three reversible oxidation and two reversible reduction peaks. Both oxidation and reduction potentials in 4a and 4b were split, which indicates the presence of effective electronic communication between two porphyrin units. Only the first oxidation potential of 4b (0.419 V) was lowered when compared with that of 4a (0.438 V). This result suggests that twisting affects only the HOMO level to decrease the HOMO−LUMO gap. The HOMO levels of monomers 1a and 1b are the same (0.50 V); therefore, the increase in the HOMO energy in 4b is primarily caused by π-twisting rather than electronic factors of the substituents. These results demonstrate the ability to control HOMO−LUMO gaps by molecular twisting. Two-photon absorption (TPA) is a third-order nonlinear optical phenomenon, and its probability of occurrence is closely related to π-electron behavior. TPA cross-section values were measured by the Z-scan technique at 1200 nm.14 The TPA cross-section value of 4a (640 GM) was somewhat lower than that of planar dimer 4c (850 GM) (Figure S9).15 We attempted to synthesize π-extended porphyrin tetramer 7. The nitration of 4a with LiNO3 afforded nitrodiporphyrin 5 in 49% yield, which was converted to aminodiporphyrin 6 in 93% yield by reduction with NaBH4 in the presence of Pd/C.16 Oxidation of 6 under the standard conditions furnished tetraporphyrin 7 in 9% yield. The use of dichloroacetic acid in place of TFA improved the yield of 7 to 24% (Scheme 2).17 In contrast to dimer 4a, six peaks of mesityl groups were observed as relatively sharp signals, which indicate that twist B
DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
This result demonstrates that π-twisting influences interchromic electronic interactions. In conclusion, we have successfully synthesized a set of highly twisted, but effectively π-conjugated, fused porphyrin oligomers through oxidative coupling of aminoporphyrins under very mild conditions. We have also demonstrated that the twist angle and the extent of dynamic motion could be controlled via the choice of substituents. Twisting of the πsurface results in reduction of the HOMO−LUMO gap and an enhancement of solubility. The present findings represent an important advance toward the long-term goal of preparing πconjugated molecular wires, whose properties can be fine-tuned through molecular twisting.
Scheme 2. Synthesis of Pyrazine-Fused Tetraporphyrin 7
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ASSOCIATED CONTENT
* Supporting Information S
Experimental details and spectral data for all new compounds. Crystallographic data (CIF files) for 1a, 4a, 4b, and 7. This material is available free of charge via the Internet at http:// pubs.acs.org.
inversion did not occur at room temperature. The helicity inversion barrier in helical polymers is known to increase in higher polymers.18 This is the first observation of a similar phenomenon in twisted π-conjugated molecules. The unidirectionality twisted structure of 7 was elucidated by X-ray diffraction analysis (Figure 3). Three slightly different
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AUTHOR INFORMATION
Corresponding Authors
[email protected] [email protected] [email protected] [email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Crystallographic data collection was performed at the BL38B1 in the SPring-8 at the BL38B1 with approval of JASRI (proposal No. 2014A1252). This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas “piSystem Figuration” (26102003) and “Science of Atomic Layers” (26107519) and the Program for Leading Graduate Schools “Integrative Graduate Education and Research in Green Natural Sciences”, MEXT, Japan. H.S. acknowledges the Yamada Science Foundation for financial support. H.I. acknowledges a Grant-in-Aid for Scientific Research (No. 24685026). This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species” (25109502) to N.K. N.K. also thanks Dr. T. Furuyama for the help in MCD measurements. The work at Yonsei University was financially supported by the Midcareer Researcher program (2005-0093839) and the Global Research Laboratory (2013K1A1A2A02050183) administered through the National Research Foundation of Korea, funded by MEST, Korea.
Figure 3. X-ray crystal structure of 7. (a) Front view and (b) perspective view. The thermal ellipsoids are scaled at 50% probability level. Mesityl groups are omitted for clarity.
conformations were observed in the crystal, with edge-to-edge twisting angles of 298°, 281°, and 295° (Figure S13). Calculated proton chemical shifts of 7 in the twisted structure were obtained by the GIAO method at the B3LYP/6-31G(d) level. A linear correlation between the calculated and observed chemical shifts indicates that tetramer 7 also adopts a twisted conformation in solution (Figure S14). The twisting angles of each two porphyrin fragment were 141°, 161°, and 145°, all of which are larger than that of 4a. The larger twisting is a reason for higher inversion energy of 7. Tetramer 7 has remarkable solubility in common organic solvents including hexane. The UV−vis absorption spectrum of 7 in CH2Cl2 is shown in Figure 2. The substantial bathochromic shift of the lowest energy band suggests the existence of effective π-conjugation over the entire molecule despite large twisting. The presence of electronic communication was also confirmed by electrochemical analysis, which exhibited multiple splitting of oxidation potentials (Figure S2). Interestingly, the average splitting in 7 (0.14 V) was smaller than that in 4a (0.23 V).
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REFERENCES
(1) (a) Haley, M. M.; Tykwinski, R. R. Carbon-Rich Compounds; Wiley-VCH: Weinheim, Germany, 2006. (b) Petrukhina, M. A.; Scott, L. T. Fragments of Fullerenes and Carbon Nanotubes: Designed Synthesis, Unusual Reactions, and Coordination Chemistry; Wiley: Hoboken, NJ, 2012. (c) Gleiter, R.; Hopf, H. Modern Cyclophane Chemistry; WileyVCH: Weinheim, Germany, 2004. (2) (a) Shen, Y.; Chen, C.-F. Chem. Rev. 2012, 112, 1463. (b) Wu, Y.T.; Siegel, J. S. Chem. Rev. 2006, 106, 4843. (3) (a) Sakurai, H.; Daiko, T.; Hirao, T. Science 2003, 301, 1878. (b) Omachi, H.; Segawa, Y.; Itami, K. Acc. Chem. Res. 2012, 45, 1378. C
DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society (4) (a) Seybold, G. Angew. Chem., Int. Ed. Engl. 1977, 16, 365. (b) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500. (5) Ding, Z.-F.; Sanchez, T.; Labouriau, A.; Iyer, S.; Larson, T.; Currier, R.; Zhao, Y.; Yang, D. J. Phys. Chem. B 2010, 114, 10337. (6) (a) Akita, M.; Hiroto, S.; Shinokubo, H. Angew. Chem., Int. Ed. 2012, 51, 2894. (b) Yokoi, H.; Wachi, N.; Hiroto, S.; Shinokubo, H. Chem. Commun. 2014, 50, 2715. (c) Goto, K.; Yamaguchi, R.; Hiroto, S.; Ueno, H.; Kawai, T.; Shinokubo, H. Angew. Chem., Int. Ed. 2012, 51, 10333. (7) (a) Pascal, R. A., Jr. Chem. Rev. 2006, 106, 4809. (b) Pradhan, A.; Dechambenoit, P.; Bock, H.; Durola, F. Angew. Chem., Int. Ed. 2011, 50, 12582. (c) Rodríguez-Lojo, D.; Pérez, D.; Peña, D.; Guitián, E. Chem. Commun. 2013, 49, 6274. (d) Yamamoto, K.; Oyamada, N.; Xia, S.; Kobayashi, Y.; Yamaguchi, M.; Maeda, H.; Nishihara, H.; Uchimaru, T.; Kwon, E. J. Am. Chem. Soc. 2013, 135, 16526. (e) Wakamiya, A.; Nishimura, H.; Fukushima, T.; Suzuki, F.; Saeki, A.; Seki, S.; Osaka, I.; Sasamori, T.; Murata, M.; Murata, Y.; Kaji, H. Angew. Chem., Int. Ed. 2014, 53, 5800. (f) Xiao, J.; Duang, H. M.; Liu, Y.; Shi, W.; Ji, L.; Li, G.; Li, S.; Liu, X.-W.; Ma, J.; Wudl, F.; Zhang, Q. Angew. Chem., Int. Ed. 2012, 51, 6094. (g) Pascal, R. A., Jr.; McMillan, W. D.; Van Engen, D. J. Am. Chem. Soc. 1986, 108, 5652. (h) Norton, J. E.; Houk, K. N. J. Am. Chem. Soc. 2005, 127, 4162. (i) Duong, H. M.; Bendikov, M.; Steiger, D.; Zhang, Q.; Sonmez, G.; Yamada, J.; Wudl, F. Org. Lett. 2003, 5, 4433. (j) Lu, J.; Ho, D. M.; Vogelaar, N. J.; Krami, C. M.; Pascal, R. A., Jr. J. Am. Chem. Soc. 2004, 126, 11168. (8) Kawasumi, K.; Zhang, Q.; Segawa, Y.; Scott, L. T.; Itami, K. Nat. Chem. 2013, 5, 739. (9) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1975, 97, 3660. (10) (a) Vicente, M. G. H.; Jaquinod, L.; Smith, K. M. Chem. Commun. 1999, 1771. (b) Anderson, H. L. Chem. Commun. 1999, 2323. (c) Arnold, D. P. Synlett 2000, 296. (d) Burrel, A. K.; Officer, D. L. Synlett 1998, 1297. (e) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57. (f) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735. (11) (a) Lü, T. X.; Reimers, J. R.; Crossley, M. J.; Hush, N. S. J. Phys. Chem. 1994, 98, 11878. (b) Tsuda, A.; Furuta, H.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 10304. (12) Haddad, R. E.; Gazeau, S.; Pécaut, J.; Marchon, J.-C.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2003, 125, 1253. (13) Muranaka, A.; Yokoyama, M.; Matsumoto, Y.; Uchiyama, M.; Tsuda, A.; Osuka, A.; Kobayashi, N. ChemPhysChem 2005, 6, 171. (14) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. J.; Van Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760. (15) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Angew. Chem., Int. Ed. 2009, 48, 3244. (16) Beavington, R.; Rees, P. A.; Burn, P. L. J. Chem. Soc., Perkin Trans. 1 1998, 2847. (17) The main byproduct in oxidation of 6 was dioxodiporphyrin 8, of which compound data were shown in the Supporting Information. (18) Ute, K.; Hirose, K.; Kashimoto, H.; Nakayama, H.; Hatada, K.; Vogl, O. Polym. J. 1993, 25, 1175.
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DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Synthesis of Highly Twisted and Fully π‑Conjugated Porphyrinic Oligomers Satoru Ito,† Satoru Hiroto,*,† Sangsu Lee,‡ Minjung Son,‡ Ichiro Hisaki,§ Takuya Yoshida,⊥ Dongho Kim,*,‡ Nagao Kobayashi,*,⊥ and Hiroshi Shinokubo*,† †
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Department of Chemistry, Yonsei University, Seoul 120-749, Korea § Department of Material and Life Science, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan ⊥ Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan ‡
S Supporting Information *
We attempted oxidative fusion of aminoporphyrin 3a with four mesityl groups, which was prepared through nitration and reduction of 1a. Oxidation of 3a with DDQ (2.0 equiv) in CHCl3 at room temperature afforded fused dimer 4a in 4% yield. After extensive optimization, we found that the oxidation of 3a in α,α,α-trifluorotoluene/trifluoroacetic acid (TFA, 20 equiv) afforded 4a in 97% yield (Scheme 1). None of the desired product was obtained without TFA. This protocol was also applied to 3b with sterically more demanding substituents to furnish 4b in 67% yield. Notably, this procedure enables the synthesis of highly congested diporphyrins 4a within 10 min even at room temperature.
ABSTRACT: Highly twisted π-conjugated molecules have been attractive but challenging targets. We report here an efficient synthesis of highly twisted diporphyrins with 126° and 136° twist angles that involves an oxidative fusion reaction of planar aminoporphyrin precursors at room temperature. Repeated amination−oxidative fusion sequences provide a unidirectionally twisted tetramer. The twisting angle of the tetramer is 298°. hree-dimensional (3D) π-conjugated molecules have received extensive attention in many chemical research areas due to their interesting structure and unique characteristics.1−3 Since the discovery of buckminsterfullerenes, synthetic chemists have devoted considerable effort to develop bottomup approaches to 3D π-conjugated molecules. Construction of 3D structures generally requires harsh conditions, such as laser irradiation, flash vacuum pyrolysis, and photochemical processes, to overcome their high strain energy.4 However, these protocols could not apply to π-conjugated functional molecules such as porphyrins, which undergo decomposition on heating and light irradiation. Aniline oxidation is a classic methodology for linking aromatic amines. However, the synthetic usefulness of this method has been overshadowed because of uncontrollable reactions to form a mixture of fused oligomers.5 In contrast, we have found that the oxidation of 2-amino-5,10,15-triarylporphyrins proceeded selectively to provide planar pyrazine-fused diporphyrins in high yields in one step.6 Herein, we report the synthesis of highly twisted diporphyrins and tetraporphyrin with very large twisting angles (136° and 298°) from planar 2aminoporphyrin precursors with more bulky substituents. Highly twisted molecules, particularly π-systems with 3D character, remain attractive but challenging targets.7 Twisting πsystems lead to the emergence of intriguing characteristics such as chiroptical properties and anisotropic crystal packing. The twisting approach to 3D π-systems has been investigated in the synthesis of oligoacenes, referred to as “twistacenes”. The maximum edge-to-edge twisting angle of the longest twistacene is 144°.7j However, additional systems with greater twist angles are needed if the specific features that are induced by molecular twisting are to be explored in detail.
T
© XXXX American Chemical Society
Scheme 1. Oxidative Dimerization of Nickel(II) 3Aminoporphyrins 3 to Diporphyrins 4a
a (a) LiNO3 (2 equiv), CHCl3/AcOH/Ac2O, 75 °C, 15 h for 1a; LiNO3 (4 equiv), CHCl3/AcOH/Ac2O, 55 °C, 4 h for 1b. (b) Sn (excess), conc. HCl, CHCl3, 60 °C, 1 h for 2a; Sn (excess), conc. HCl, CHCl3, 60 °C, 3 h for 2b. (c) DDQ (2 equiv), TFA (20 equiv), PhCF3, RT, 10 min for 3a; DDQ (2 equiv), TFA (20 equiv), PhCF3, RT, 2 h for 3b.
Received: November 20, 2014
A
DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society X-ray diffraction analysis of the products revealed that the present oxidative fusion reaction transformed 2D planar precursors into 3D twisted π-systems. The structures of the fused diporphyrins 4a and 4b are shown in Figure 1 and Figure
Figure 1. X-ray crystal structure of dimer 4a (a) side and (b) front views. Mesityl groups are omitted for clarity. The thermal ellipsoids are scaled at 50% probability level.
S11, respectively. Interestingly, both 4a and 4b exhibit highly twisted conformations in which the central pyrazine ring is also twisted. The twist angles of the pyrazine rings in 4a (13.01(9)°) and 4b (13.18(8)°) are nearly the same. While 1a is completely planar (Figure S10), the porphyrin units in 4a and 4b adopt ruffled conformations. The two porphyrin units are twisted in the same direction, and the edge-to-edge twisting angles in 4a and 4b are 125.6(8)° and 136.3(7)°, respectively. Meanwhile, 4a and 4b demonstrate unusually high solubility even in hexane.8 This fact demonstrates that deformation of πsurfaces leads to significant enhancement of solubility. This is remarkable since porphyrin oligomers typically suffer from low solubility. Different dynamic behaviors between 4a and 4b were observed in 1H NMR analysis. At room temperature, 4a exhibited two broad signals due to protons at the 3- and 5positions of the mesityl groups. On the other hand, four aryl proton peaks were observed in 4b. This difference is likely due to the rapid twist inversion of 4a compared with the relatively stable twisting of 4b at room temperature. In principle, the aryl proton signals should appear as four peaks in the twisted conformation. However, these peaks are interconvertible and merged into two peaks by twist inversion. This hypothesis was confirmed by variable temperature NMR analysis; four aryl proton signals in 4b became broader at higher temperatures and coalesced at 100 °C (Figure S5). Because of the high rotation barrier of mesityl groups, the signal broadening can be attributed to the helicity inversion.9 In 1H NMR spectra, pyrrole proton signals of 4b were upfield shifted as compared to 4a, while chemical shifts of monomers 1a and 1b were almost the same. This indicates that the π-twisting significantly influences the ring current of the porphyrin unit. The lower NICS(0) values of 4a than 4b also support substantially weakened aromaticity of the porphyrin ring (Figure S15). We expected that the subtle effect of molecular twisting could be detectable on the basis of change in interporphyrinic interactions in the twisted diporphyrins.10 Figure 2 shows the UV−vis absorption spectra of 1a, 1b, 4a, and 4b in CH2Cl2. Both dimers 4a and 4b exhibit bathochromic shift and broadening of the spectra compared with the corresponding monomers 1a and 1b, which indicates effective expansion of πconjugation despite their largely twisted structures.11 Compared with 4a, 4b shows slight bathochromic shift (51 cm−1) of the lowest energy band.12 The spectrum of 4a was further assigned by magnetic circular dichroism (MCD) spectral analysis. MCD signals for two bands at 622 and 585 nm
Figure 2. Magnetic circular dichroism (MCD) (top) and UV−vis absorption (bottom) spectra of 1a (black dash line), 1b (red dash line), 4a (black solid line), 4b (red solid line), and 7 (blue solid line) in CH2Cl2. The inset displays the enlarged absorption spectra.
showed oppositely signed Faraday B terms, which indicate the presence of two differently polarized transitions. Considering their optically forbidden character, these bands can be assigned as split Q-bands directed to two different axes (long and short axes, respectively). This type of separation of two Q-bands is often observed for fused mulitiporphyrins with effective πconjugation.13 To gain further insight into their electronic transitions, electrochemical analysis was performed. The results of cyclic voltammetry for 4a and 4b are summarized in Table S1. Both compounds showed three reversible oxidation and two reversible reduction peaks. Both oxidation and reduction potentials in 4a and 4b were split, which indicates the presence of effective electronic communication between two porphyrin units. Only the first oxidation potential of 4b (0.419 V) was lowered when compared with that of 4a (0.438 V). This result suggests that twisting affects only the HOMO level to decrease the HOMO−LUMO gap. The HOMO levels of monomers 1a and 1b are the same (0.50 V); therefore, the increase in the HOMO energy in 4b is primarily caused by π-twisting rather than electronic factors of the substituents. These results demonstrate the ability to control HOMO−LUMO gaps by molecular twisting. Two-photon absorption (TPA) is a third-order nonlinear optical phenomenon, and its probability of occurrence is closely related to π-electron behavior. TPA cross-section values were measured by the Z-scan technique at 1200 nm.14 The TPA cross-section value of 4a (640 GM) was somewhat lower than that of planar dimer 4c (850 GM) (Figure S9).15 We attempted to synthesize π-extended porphyrin tetramer 7. The nitration of 4a with LiNO3 afforded nitrodiporphyrin 5 in 49% yield, which was converted to aminodiporphyrin 6 in 93% yield by reduction with NaBH4 in the presence of Pd/C.16 Oxidation of 6 under the standard conditions furnished tetraporphyrin 7 in 9% yield. The use of dichloroacetic acid in place of TFA improved the yield of 7 to 24% (Scheme 2).17 In contrast to dimer 4a, six peaks of mesityl groups were observed as relatively sharp signals, which indicate that twist B
DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
This result demonstrates that π-twisting influences interchromic electronic interactions. In conclusion, we have successfully synthesized a set of highly twisted, but effectively π-conjugated, fused porphyrin oligomers through oxidative coupling of aminoporphyrins under very mild conditions. We have also demonstrated that the twist angle and the extent of dynamic motion could be controlled via the choice of substituents. Twisting of the πsurface results in reduction of the HOMO−LUMO gap and an enhancement of solubility. The present findings represent an important advance toward the long-term goal of preparing πconjugated molecular wires, whose properties can be fine-tuned through molecular twisting.
Scheme 2. Synthesis of Pyrazine-Fused Tetraporphyrin 7
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ASSOCIATED CONTENT
* Supporting Information S
Experimental details and spectral data for all new compounds. Crystallographic data (CIF files) for 1a, 4a, 4b, and 7. This material is available free of charge via the Internet at http:// pubs.acs.org.
inversion did not occur at room temperature. The helicity inversion barrier in helical polymers is known to increase in higher polymers.18 This is the first observation of a similar phenomenon in twisted π-conjugated molecules. The unidirectionality twisted structure of 7 was elucidated by X-ray diffraction analysis (Figure 3). Three slightly different
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AUTHOR INFORMATION
Corresponding Authors
[email protected] [email protected] [email protected] [email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Crystallographic data collection was performed at the BL38B1 in the SPring-8 at the BL38B1 with approval of JASRI (proposal No. 2014A1252). This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas “piSystem Figuration” (26102003) and “Science of Atomic Layers” (26107519) and the Program for Leading Graduate Schools “Integrative Graduate Education and Research in Green Natural Sciences”, MEXT, Japan. H.S. acknowledges the Yamada Science Foundation for financial support. H.I. acknowledges a Grant-in-Aid for Scientific Research (No. 24685026). This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species” (25109502) to N.K. N.K. also thanks Dr. T. Furuyama for the help in MCD measurements. The work at Yonsei University was financially supported by the Midcareer Researcher program (2005-0093839) and the Global Research Laboratory (2013K1A1A2A02050183) administered through the National Research Foundation of Korea, funded by MEST, Korea.
Figure 3. X-ray crystal structure of 7. (a) Front view and (b) perspective view. The thermal ellipsoids are scaled at 50% probability level. Mesityl groups are omitted for clarity.
conformations were observed in the crystal, with edge-to-edge twisting angles of 298°, 281°, and 295° (Figure S13). Calculated proton chemical shifts of 7 in the twisted structure were obtained by the GIAO method at the B3LYP/6-31G(d) level. A linear correlation between the calculated and observed chemical shifts indicates that tetramer 7 also adopts a twisted conformation in solution (Figure S14). The twisting angles of each two porphyrin fragment were 141°, 161°, and 145°, all of which are larger than that of 4a. The larger twisting is a reason for higher inversion energy of 7. Tetramer 7 has remarkable solubility in common organic solvents including hexane. The UV−vis absorption spectrum of 7 in CH2Cl2 is shown in Figure 2. The substantial bathochromic shift of the lowest energy band suggests the existence of effective π-conjugation over the entire molecule despite large twisting. The presence of electronic communication was also confirmed by electrochemical analysis, which exhibited multiple splitting of oxidation potentials (Figure S2). Interestingly, the average splitting in 7 (0.14 V) was smaller than that in 4a (0.23 V).
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REFERENCES
(1) (a) Haley, M. M.; Tykwinski, R. R. Carbon-Rich Compounds; Wiley-VCH: Weinheim, Germany, 2006. (b) Petrukhina, M. A.; Scott, L. T. Fragments of Fullerenes and Carbon Nanotubes: Designed Synthesis, Unusual Reactions, and Coordination Chemistry; Wiley: Hoboken, NJ, 2012. (c) Gleiter, R.; Hopf, H. Modern Cyclophane Chemistry; WileyVCH: Weinheim, Germany, 2004. (2) (a) Shen, Y.; Chen, C.-F. Chem. Rev. 2012, 112, 1463. (b) Wu, Y.T.; Siegel, J. S. Chem. Rev. 2006, 106, 4843. (3) (a) Sakurai, H.; Daiko, T.; Hirao, T. Science 2003, 301, 1878. (b) Omachi, H.; Segawa, Y.; Itami, K. Acc. Chem. Res. 2012, 45, 1378. C
DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society (4) (a) Seybold, G. Angew. Chem., Int. Ed. Engl. 1977, 16, 365. (b) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500. (5) Ding, Z.-F.; Sanchez, T.; Labouriau, A.; Iyer, S.; Larson, T.; Currier, R.; Zhao, Y.; Yang, D. J. Phys. Chem. B 2010, 114, 10337. (6) (a) Akita, M.; Hiroto, S.; Shinokubo, H. Angew. Chem., Int. Ed. 2012, 51, 2894. (b) Yokoi, H.; Wachi, N.; Hiroto, S.; Shinokubo, H. Chem. Commun. 2014, 50, 2715. (c) Goto, K.; Yamaguchi, R.; Hiroto, S.; Ueno, H.; Kawai, T.; Shinokubo, H. Angew. Chem., Int. Ed. 2012, 51, 10333. (7) (a) Pascal, R. A., Jr. Chem. Rev. 2006, 106, 4809. (b) Pradhan, A.; Dechambenoit, P.; Bock, H.; Durola, F. Angew. Chem., Int. Ed. 2011, 50, 12582. (c) Rodríguez-Lojo, D.; Pérez, D.; Peña, D.; Guitián, E. Chem. Commun. 2013, 49, 6274. (d) Yamamoto, K.; Oyamada, N.; Xia, S.; Kobayashi, Y.; Yamaguchi, M.; Maeda, H.; Nishihara, H.; Uchimaru, T.; Kwon, E. J. Am. Chem. Soc. 2013, 135, 16526. (e) Wakamiya, A.; Nishimura, H.; Fukushima, T.; Suzuki, F.; Saeki, A.; Seki, S.; Osaka, I.; Sasamori, T.; Murata, M.; Murata, Y.; Kaji, H. Angew. Chem., Int. Ed. 2014, 53, 5800. (f) Xiao, J.; Duang, H. M.; Liu, Y.; Shi, W.; Ji, L.; Li, G.; Li, S.; Liu, X.-W.; Ma, J.; Wudl, F.; Zhang, Q. Angew. Chem., Int. Ed. 2012, 51, 6094. (g) Pascal, R. A., Jr.; McMillan, W. D.; Van Engen, D. J. Am. Chem. Soc. 1986, 108, 5652. (h) Norton, J. E.; Houk, K. N. J. Am. Chem. Soc. 2005, 127, 4162. (i) Duong, H. M.; Bendikov, M.; Steiger, D.; Zhang, Q.; Sonmez, G.; Yamada, J.; Wudl, F. Org. Lett. 2003, 5, 4433. (j) Lu, J.; Ho, D. M.; Vogelaar, N. J.; Krami, C. M.; Pascal, R. A., Jr. J. Am. Chem. Soc. 2004, 126, 11168. (8) Kawasumi, K.; Zhang, Q.; Segawa, Y.; Scott, L. T.; Itami, K. Nat. Chem. 2013, 5, 739. (9) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1975, 97, 3660. (10) (a) Vicente, M. G. H.; Jaquinod, L.; Smith, K. M. Chem. Commun. 1999, 1771. (b) Anderson, H. L. Chem. Commun. 1999, 2323. (c) Arnold, D. P. Synlett 2000, 296. (d) Burrel, A. K.; Officer, D. L. Synlett 1998, 1297. (e) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57. (f) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735. (11) (a) Lü, T. X.; Reimers, J. R.; Crossley, M. J.; Hush, N. S. J. Phys. Chem. 1994, 98, 11878. (b) Tsuda, A.; Furuta, H.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 10304. (12) Haddad, R. E.; Gazeau, S.; Pécaut, J.; Marchon, J.-C.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2003, 125, 1253. (13) Muranaka, A.; Yokoyama, M.; Matsumoto, Y.; Uchiyama, M.; Tsuda, A.; Osuka, A.; Kobayashi, N. ChemPhysChem 2005, 6, 171. (14) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. J.; Van Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760. (15) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Angew. Chem., Int. Ed. 2009, 48, 3244. (16) Beavington, R.; Rees, P. A.; Burn, P. L. J. Chem. Soc., Perkin Trans. 1 1998, 2847. (17) The main byproduct in oxidation of 6 was dioxodiporphyrin 8, of which compound data were shown in the Supporting Information. (18) Ute, K.; Hirose, K.; Kashimoto, H.; Nakayama, H.; Hatada, K.; Vogl, O. Polym. J. 1993, 25, 1175.
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DOI: 10.1021/ja511905f J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
